Bul­letin / Cut­ting edge

Fast-spin­ning as­ter­oid sends plumes of par­ti­cles into space

Sky at Night Magazine - - CONTENTS - www.as­ter­oid­mis­sion.org

As­ter­oid Bennu

is prov­ing to be an enig­matic body for planetary sci­en­tists ex­plor­ing with NASA’s OSIRIS-REx (Ori­gins, Spec­tral In­ter­pre­ta­tion, Re­source Iden­ti­fca­tion, Se­cu­rity-Re­golith Ex­plorer) space­craft. They have dis­cov­ered that it is eject­ing plumes of par­ti­cles from the sur­face into space.

The frst close-up pic­tures re­vealed the as­ter­oid’s sur­face is lit­tered with rocks and boul­ders, leav­ing mis­sion plan­ners strug­gling to fnd a spot to touch down next year and take a sam­ple to re­turn to Earth in 2023. But the ejec­tion of jets of par­ti­cles, ob­served since early Jan­uary, was com­pletely un­ex­pected.

“The dis­cov­ery of plumes is one of the big­gest sur­prises of my ca­reer,” says prin­ci­pal in­ves­ti­ga­tor Dante Lau­retta, of the Univer­sity of Ari­zona.

Some par­ti­cles were ob­served to or­bit Bennu as satel­lites be­fore fall­ing back to the sur­face. None of the plumes are seen as a threat to the probe as it or­bits at a dis­tance of around 1.6km.

The ejected plumes are un­like those from comets as they are warmed by the Sun. A clue to their cause may come from the bal­ance that ex­ists be­tween Bennu’s weak grav­i­ta­tional pull and its spin, which gives rocks enough en­ergy to es­cape into space. “When you spin this guy up, you cre­ate a com­pe­ti­tion be­tween the grav­ity that’s hold­ing you down and the cen­trifu­gal ac­cel­er­a­tion, which is try­ing to throw you off,” says Daniel Scheeres, of the mis­sion’s ra­dio sci­ence team at the Univer­sity of Colorado Boulder.

Bennu’s or­bit brings it close to Earth as a po­ten­tially haz­ardous ob­ject. Dust par­ti­cles ejected in the past may even pro­duce a mi­nor me­teor shower.

NASA’s OSIRIS- REx has dis­cov­ered par­ti­cles be­ing ejected from as­ter­oid Bennu, and in­set, its rocky sur­face

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